Developer



ened, changing to a dense black; the background areas remained white. The thin coating of solution remaining on the sheet was not apparent as a liquid layer and the water content was soon volatilized and removed,

A portion of the sheet having an average image density at an image area somewhat greater than 1A x 1A inch was tested for light-reflectance as compared with the unexposed copy-paper, by means of a Densichron optical density measuring instrument. Such an instrument measures the percentage of incident light reflected from a test surface in comparison with that reflected from a standard surface. With the instrument set to give a zero percent absorption reading (i.e. 100% reflection) for the unexposed sheet, a reading of 69 units was obtained from the image area, meaning that 69% of the light normally reflected from the unprinted copy-paper was absorbed. It may be shown that readings down to about 40-50 units in the image areas and -10 units (i.e., 90l00% reflection) in the background areas indicate highly acceptable graphic representations. Total absorption would be equivalent to 100 units on the scale indicated.

The print was stored for one week in a humidity chamber at room temperature and 98% relative humidity. The image density remained at 69 units. The background area likewise remained well below 10 units. Since this was found to be generally true in each of the examples, background values will not be reported hereinafter other than for exceptional cases.

Reducing the concentration of hydrated nickel acetate to 2% while maintaining the silver nitrate at 0.5% produced image areas of 63 unit initial density and showing no decrease on one week aging at 98% R.H.

In a comparative test, a solution of ten parts of the hydrated nickel acetate in water to make 100 parts, but in the absence of the silver nitrate, formed an image having an initial density of only 48 units. After one week at 98% R.H. the density had fallen to 41 units.

In another comparative test, a 0.5% solution of silver nitrate formed a yellowish brown image having a density of 40 units. After one week at 98% R.H. the reading was 44 units. Substantially identical results were obtained using concentrations of one and two percent of silver nitrate in Water. At higher concentrations, g. at 10% of silver nitrate, visual inspection indicated no increase, and possibly a slight decrease in density as compared with that obtained in the neighborhood of one percent. The background areas were significantly darkened where the silver nitrate was the only salt present.

Example 2 A solution of three parts by weight of hydrated cadmium nitrate (Cd(NO3)2-4H2O), one-half part of tartar emetic (antimonyl potassium tartrate, K(SbO)C4H4O61/2H2O) and one-half part of silver nitrate in water suflieient to make 100 parts Was employed as described under Example 1 in the electrolytic development of images. Dense black image areas were produced. The image areas did not observably decrease in density on storage for more than one week at 98% R.H. and normal room temperature.

A solution of 2% hydrated cadmium nitrate and 1% tartar emetic produced an initial density of 72 units, dropping to 60 units in a first week at 98% R.H. and to less than 50 units in a second week at the high humidity. The addition of 0.5% silver nitrate increased the initial density obtained to 82 units, and after one week in the test chamber the reading remained at 80 units.

Image areas produced with a solution of 2% of the hydrated cadmium nitrate were initially black but disappeared completely within about one hour at 98% R.H.

Initially black images produced with a 5% solution of tartar emetic disappeared within one week at 98% R.H. Density readings taken on the image areas were 42 units as 4 rst developed, dropping to zero during the week of aging. The addition of 1% silver nitrate to this solution resulted in image areas having an initial density of 72 units, dropping to 66 units after one week in the test chamber.

Example 3 Solutions at 3% and at 10% concentrations of nickel nitrate in Water produced no visible image when tested in the electrolytic development procedure as described in Example l. The addition of small proportions of silver nitrate to the 10% solution resulted in formation of dense black images having an initial density, as measured on the Densichron instrument, of 73 units. After one week at 98% R.H. the density reading was 73 units.

Example 4 Ten percent of hydrated magnesium nitrate and 5% of tartar emetic gave an electrolytic developer solution with which image areas were produced having an initial density of 61 units and a density after one week in the test cell of 44 units. The addition of 1% of silver nitrate increased the initial density to 69 units, and this value was retained in the aging test.

Example 6 A solution of 2% nickel acetate hydrate and 0.5% silver acetate produced an image area of 69 unit initial density; after one week in the test cell the density was 64 units.

Example 7 A solution of 2% nickel sulfate and 0.5% silver sulfate similarly provided an initial density of 72 units; after one week in the test cell the density was 67 units.

Example 8 A 0.5 percent concentration of (HAuCl4-3H2O) when applied as an electrolytic developer produced an image having an initial density of 37 units. A 2.0% solution of antimonyl fluoride (SbF3) similarly provided an image density of 37 units. A mixture of the two solutions produced an image area having a density of 56 units, and the high density level was permanently retained.

Electrolytic development is found to proceed at a more rapid rate with the mixed salt developers including silver ion or the like than in the absence of such material, as well as with the formation of improved images; and the mixed solutions also require somewhat less intensity or time of illumination of the copy-sheet with the light-image.

Although the proportion of silver salt may be varied considerably, and although extremely small trace amounts have produced an observable effect, it is generally found desirable to include at least about 0.01% by weight and in no case greater than about 5%. Excellent prints are obtained with concentrations in the neighborhood of 0.5 percent. Similarly, the concentration of the base metal salt may vary from as low as 1% or even somewhat lower up to 10% or 15% or somewhat higher. At too low a concentration the amount of metal ion is insufficient to produce the required density of image; at excessive concentrations, the amount of salts deposited on the copy-paper may be so high as to cause darkening of background areas. Excessive proportions of noble metal ion are economically undesirable and tend to cause deposition of such metal at background areas. Ordinarily the amount of silver ion or the like is therefore from trace amounts up to not more than a minor proportion, in terms of molar ratio, of the total amount of platable metal ion. l

Even trace amounts of silver ion are diicult to obtain in the case of diiiicultly soluble silver salts. In such cases it has been found helpful to add sodium thiosulfate in amount sufficient to dissolve an adequate amount of the silver salt.

Although images may be produced with solutions containing soluble salts of any of the metals below magnesium in the electromotive series when properly handled, it is found that salts of cadmium, nickel, zinc, copper and cobalt are in general most satisfactory. Among the remainder, ferric ion is undesirable since it is unstable when exposed to sunlight; and ferrous ion is incompatible with silver ion, causing reduction of the latter and deposition of metallic silver. Systems which are unstable on exposure to light or because of inter-reaction in this way are unde sirable and are excluded.

Aqueous systems have been illustrated in the specific examples and are preferred; but ionizing solvents other than water are also useful. Solutions have, for example, Y

been prepared in ethyl alcohol, Glycerine and formamide are also effective, but since they are essentially non-volatile they must subsequently remain on or in the sheet and their use is therefore not recommended.

What is claimed is as follows:

1. The process of producing long-lasting dense imageforming deposits on light-exposed areas of strongly photoconductive photosensitive copy-paper having a photoconductive coating on an electrically conductive backing, said copy-paper being capable of electrolytic image reproduction thereon, which comprises electrolyzing at the conductive light-exposed areas a stable liquid electrolytic developer solution consisting essentially of, in solution in an ionizing solvent, from about 0.1 to about Weight percent of a silver salt and from about 1 to about 15 weight percent of a soluble salt of a platable metal selected from the group consisting of cadmium, nickel, zinc, copper and cobalt, the silver concentration being a minor amount, in terms of molar ratio, of the total amount of said platable metal, said electrically conductive backing being connected as cathode.

2. The process of producing a long-lasting visible reproduction of a graphic original on strongly photoconductive zinc oxide coated copy-paper having a photoconductive coating on an electrically conductive backing, said copypaper being capable of electrolytic image reproduction thereon, comprising exposing the coated surface to a light image corresponding to said graphic original and elecveloper solution consisting essentially of, in solution in an ionizing solvent, from about 0.1 to about 5 weight percent of a silver salt and from about 1 to about 15 weight percent of a soluble salt of a platable metal selected from the group consisting of cadmium, nickel, zinc, copper, and cobalt, the silver concentration being a minor amount, in terms of molar ratio, of the total amount of said platable metal, said electrically conductive backing being con-v nected as cathode.

3. The process of claim 1 in which the platable metal is cadmium.

4. The process of claim 1 in which the platable metal is nickel.

5. The process of claim 1 in which the platable metal 1s zinc.

6. The process of claim 1 in which the platable metal is copper.

7. The process of claim 1 in which the platable metai is cobalt.

8. The process of claim 2 in which the platable metal is cadmium.

9. The process of claim 2 in which the platable metal is nickel. 1.0. The process of claim 2 in which the platable metal 1s zinc. n l1. The process of claim 2 in which the platable metal 1s copper.

12. The process of claim 2 in which the platable metal is cobalt.

References Cited in the le of this patent UNITED STATES PATENTS 1,782,092 Gray et al Nov. 18, 1930 1,902,213 Brockway Mar. 21, 1933 2,083,249 Thomson June 8, 1937 2,186,859 Digby Jan. 9, 1940 2,259,270 Ryder Oct. 14, 1941 2,443,119 Rubin Iune 8, 1948 2,555,375 Ruemmler June 5, 1951 2,660,554 Ostrow Nov. 24, 1953 2,692,190 Pritikin Oct. 19, 1954 2,757,134 Turner July 31, 1956 2,777,810 Ostrow Jan. 15, 1957 FOREIGN PATENTS 151,971 Germany Dec. 8, 1902 215,754 Australia June 23, 1958 OTHER REFERENCES Sanigar: Transactions Electrochemical Society, vol. 59 (1931), pages 3l5316. 

1. THE PROCESS OF PRODUCING LONG-LASHING DENSE IMAGEFORMING DEPOSITS ON LIGHT-EXPOSED AREAS OF STRONGLY PHOTOCONDUCTIVE PHOTOSENSITIVE COPY-PAPER HAVING A PHOTOCONDUCTIVE COATING ON AN ELECTRICALLY CONDUCTIVE BACKING, SAID COPY PAPER BEING CAPABLE OF ELECTRICALLY IMAGE REPRODUCTION THEREON, WHICH COMPRISES ELECTROLYZING AT THE CONDUCTIVE LIGHT-EXPOSED AREAS A STABLE LIQUID ELECTROLYTIC DEVELOPER SOLUTION CONSISTING ESSENTIALLY OF, IN SOLUTION IN AN LONIZING SOLVENT, FROM ABOUT 0.1 TO ABOUT 5 WEIGHT PERCENT OF A SILVER SALT AND FROM ABOUT 1 TO ABOUT 15 WEIGHT PERCENT OF A SOLUBLE SALT OF A PLATABLE METAL SELECTED FROM THE GROUP CONSISTING OF CADMIUM, NICKEL, ZINC, COPPER AND COBALT, THE SILVER CONCENTRATION BEING A MINOR AMOUNT, IN TERMS OF MOLAR RATIO, OF THE TOTAL AMOUNT OF SAID PLATABLE METAL, SAID ELECTRICALLY CONDUCTIVE BACKING BEING CONNECTED AS CATHODE. 